Interference or noise estimation plays a key role in the performance of a cellular network. In the case of an LTE (Long Term Evolution) network comprising a number of network nodes (such as radio base stations) and wireless devices (such as instances of user equipment, UE), downlink interference estimation is typically involved in two different processes. The first one is the demodulation of the useful downlink signal from a serving network node. The received signal is usually a mix of the useful signal which carries the information from the serving network node, and interfering signals originating from other network nodes. The receiver attempts to obtain an estimation of the interfering network nodes in order to estimate the useful signal. The more accurate the interference estimation is, the better the reception of useful signal is. The second process is channel quality estimation (CQI). Here, each wireless device should estimate and report to the transmitter the experienced interference. The last one is used in link adaptation (LA) for selecting appropriate modulation and coding rate as well as precoder and rank for the oncoming transmissions. The more accurate the interference estimation is, the better the LA works leading to successful reception. From the above it is obvious that accurate interference estimation can improve data reception at the UE side as well as the overall network performance.
In LTE Release 8, downlink interference estimation for both demodulation and CQI estimation is based on a set of resource elements which are called CRSs (Cell specific Reference Signals). The CRSs are regularly transmitted on every subframe by each network node. They have a predefined density on the frequency domain and thus occupy specific places within a resource block. CRSs are always transmitted from each cell independent of whether the cell is transmitting data or not. This mismatch between data and CRS transmissions leads to a big mismatch between the estimated and the actual data interference.
In LTE Release ii, interference estimation for CQI estimation is also possible based on a set of resource elements which are called IMRs (Interference Measurement Resources). The IMRs are muted resource elements, in which no data or signalling is scheduled, which are used for interference estimation of surrounding cells. They occur in predefined time cycles with a fixed frequency density according to an IMR configuration. The IMRs of a specific cell are thus able to collide with normal data transmissions of other cells with different IMR configurations. Compared to CRS, the mismatch between estimated and actual interference is reduced when the IMRs of a network node collide with actual data transmissions of other cells. In addition, IMRs are muted resource elements thus avoiding the impact of own useful signal on interference estimation. Nevertheless, there can still be a bias in the estimation introduced by cells that reuse the same IMR configuration and thus mute transmissions on the same resource elements. In that case, the wireless device is unable to measure interference from such cells which might lead to interference underestimation. This indicates also that IMR configurations should be carefully planned across the network in order to capture accurate interference levels. Planning imposes an extra constraint on the flexibility of using IMRs. Especially in case of massive or super dense deployments of network nodes, shortage of available IMR configurations might limit performance of interference estimation.